Method of melting glass in a platinum container in a controlled atmosphere

ABSTRACT

This invention provides methods for melting glass laser compositions to provide glass laser rods with a minimum of noble metal inclusions and a minimum of dissolved noble metal content. The process comprises melting the laser glass in a platinum container in a controlled reducing atmosphere using a buffered gas mixture, such as CO2/CO in which the partial pressure of oxygen is at a minimum level that is equivalent to a range of about 10 8.6 to 10 38 atmospheres for a temperature range of about 981*F. to 3,321*F. for a silicate glass. A method also is provided for melting glass to make high-quality glass articles in a noble metal container or a lined container in which the container or the liner therefore has at least 50% by weight of platinum without any attack on the container by the unwanted formation of a noble metal alloy from the glass melt during melting. The method comprises heating molten glass in the platinum container, providing a reducing atmosphere in which the partial pressure of oxygen is no greater than about 10 3 atmospheres with a buffered gas mixture, and controlling the reducing atmosphere by maintaining the partial pressure of oxygen above the amount at which oxides of an element such as SiO2, Li2O and Na2O are reduced to the element such as Si, Li or Na, so that the element does not react with the platinum metal to form a platinum-Si alloy.

United States Patent [1 1 Dietz et al.

[ 1 METHOD OF MELTING GLASS IN A PLATINUM CONTAINER IN A CONTROLLEDATMOSPHERE 751 Inventors: Earl D. Dietz; Paul R. Wengert,

both of Toledo, Ohio [73] Assignee: Owens-Illinois, Inc., Toledo, Ohio[21] Appl. No.: 377,883

Related U.S. Application Data [63] Continuation-in-part of Ser, Nos.177,095, Sept. 1, 1971, abandoned, and Ser. No. 177,090, Sept. 1, 1971,Pat. No. 3,837,828. r

OTHER PUBLICATIONS Abstract, American Ceramic Soc. Bull.,- Sept. 1970,p. 820, 13G70f. Contamination of Glass by Platinum. Journal ofNon-crystalline Solids, 6( 1971 Volume 6, No. 4, Nov.Dec. 1971, pp.294-306.

1*Mar. 18, 1975 Primary Examiner-S. Leon Bashore AssistantE.raminerFrank W. Miga Attorney, Agent, or FirmRichard D. Heberling; E..1. Holler [57] ABSTRACT This invention provides methods for meltingglass laser compositions to provide glass laser rods with a minimum ofnoble metal inclusions and a minimum of dissolved noble metal content.The process comprises melting the laser glassin a platinum container ina controlled reducing atmosphere using a buffered gas mixture, such asCO /CO in which the partial pressure of oxygen is at a minimum levelthat is equivalent to a range of about 10 to 10* atmospheres for atemperature range of about 981F. to 3,321F. for a silicate glass. Amethod also is provided for melting glass to make high-quality glassarticles in a noble metal .container or a lined container in which thecontainer or the liner therefore has at least 50% by weight of platinumwithout any attack on the container by the unwanted formation of a noblemetal alloy from the glass melt during melting. The method comprisesheating molten glass in the platinum container, providing a reducingatmosphere in which the partial pressure of oxygen is no greater thanabout 10 atmospheres with a buffered gas mixture, and controlling thereducing atmosphere by maintaining the partial pressure of oxygen abovethe amount atwhich oxides of an element such as SiO U 0 and Na O arereduced to the element such as Si, Li or Na, so that the element doesnot react with the platinum metal to form a platinum-Si alloy.

13 Claims, N0 Drawings METHOD OF MELTING GLASS IN A PLATINUM CONTAINERIN A CONTROLLED ATMOSPHERE The invention here describedwas made in thecourse of a contract with the United States Government.

CROSS-REFERENCES TO RELATED APPLICATIONS This application is acontinuation-in-part application of patent application Ser. No. 177,095,filed Sept. 1, l97l now abandoned and a continuation-in-part of patentapplication Ser. No. 177,090, filed Sept. l, 1971 now US. Pat. No.3,837,828.

THE INVENTION This invention relates to a method of melting glass toprovide high-quality glass articles, including glass laser articles, byheating molten glass in a noble metal container containing at leastabout 50% by weight of platinum in an atmosphere that is controlled bymaintaining the partial pressure of oxygen above the amount at whichtheoxides of a glass-forming element in the glass are reduced to theelement which, in turn, would react with the noble metal to thus attackthe container.

The invention also relates to a method of melting glass lasercompositions, preferably lithia-calciaalumina silicate glasses, toprovide high-quality glass laser rods by melting the glass in a noblemetal container containing at least about 50 percent by weight ofplatinum in an atmosphere that is reducing and that is controlled bymaintaining the partial pressure of oxygen at a minimum level that isequivalent to a range of about to lo atmospheres for a silicate glass.The controlling of the reduced atmosphere reduces the amount of noblemetal inclusions in the glass laser rod, reduces the amount of dissolvednoble metal in the resultant rod, and eliminates attack of the noblemetal container.

It is desirous to melt glass to provide high-quality articles in aplatinum container or a container that is at least 50% by weightplatinum, in addition to other noble metals, without attack on thecontainer.

It is highly desirable to melt the laser glass compositions to providevery high-quality glass laser rods in a noble metal container having amajor proportion of platinum without any undesirable amount of noblemetal inclusions, such as platinum inclusions, and a minimum ofdissolved platinum content in the rod.

It is an object of the present invention to provide a method of meltingglass to provide high-quality glass articles in a noble metal containerhaving at least 50% by weight of platinum without any attack on thecontainer by controlling the atmosphere above the melt and maintainingthe partial pressure of oxygen above the melt above a minimum amount,whereby above such minimum amounts the oxides of the glass-formingelement of the glass are reduced which element, in turn, would reactwith the platinum to attack the container.

It is an object of the present invention to provide a method of meltingglass laser compositions with a minimum of platinum and noble metalinclusions and a minimum of dissolved platinum content, in which theprocess comprises melting the laser glass composition in a noble metalcontainer having at least 50% by weight of platinum in a controlledreducing atmosphere using a buffered gas mixture in which the partialpressure of oxygen is at a minimum level that is equivalent to a rangeof about l0' to 10 atmospheres for a silicate glass.

It is an object of the present invention to provide a method of meltingglass to provide high-quality glass articles in a platinum containerwith no attack on the platinum container, the method comprising heatingmolten glass in a platinum-containing container and controlling theatmosphere above the melt with a buffered gas mixture by maintaining thepartial pressure of oxygen above the melt so that the partial pressureof oxygen is no greater than about 10 atmospheres, and so that the metaloxides in the glass are not reduced to the metal element, which metalelement would react with platinum to form a metal-platinum alloy that isdeleterious to the container.

It is an object of the present invention to provide a method of meltingglass to provide high-quality articles, such as a lithium-silicate glasscomposition, in a platinum container without attack on the platinumcontainer and without deleterious platinum inclusions in the glass.

It is an object of the present invention to melt glass lasercompositions, preferably those of a lithiumsilicate glass system, with aminimum of platinum inclusions and a minimum of dissolved platinumcontent in the rod; the process comprising melting the glass in aplatinum metal container in a controlled reducing atmosphere in whichthe partial pressure of oxygen preferably is at a minimum level that is'equivalent to a range of about 10 to 10" atmospheres for a temperaturerange of about 981F. to 3,32 1F. for a silicate glass; thepartialpressure ofioxygen of 10 atmospheres being a minimum at the lowtemperature of about 980F. and the partial pressure of 10 being aminimum at the high temperature of about 3,320F., the process producingthe aforementioned outstanding laser rod or disc with a minimum ofplatinum inclusions and a minimum of dissolved platinum content.

It is an object of the present invention to provide a method of meltinglithia-calcia-alumino silicate glass compositions to provide a glass rodwith a minimum of platinum inclusions and dissolved platinum, theprocess comprising melting the laser glass composition in a controlledatmosphere, using a buffered gas mixture such as a mixture of CO and CO,in which the partial pressure of oxygen is between about 10 and 10atmospheres at 2,600F. to produce the high-quality glass rod and at thesame time, avoid attack on the platinum container.

These and other objects will be apparent from the specification thatfollows and the appended claims.

The present invention provides a method of melting glass to providehigh-quality glass articles in a noble metal container containing atleast percent by weight of platinum, (the balance of the noble metalbeing iridium, palladium, rhodium, osmium, gold, silver, Re, andpreferably platinum) without any attack on the noble metal container bythe formation of a noble metal alloy from the glass melt, the methodcomprising heating molten glass in the container in a reducingatmosphere in which the partial pressure of oxygen is no greater thanabout 10 atmospheres with a buffered gas mixture, such as CO /CO. In theabove described method, the atmosphere is controlled by maintaining thepartial pressure of oxygen above the amount at which the oxides of aglass-forming element in the glass are reduced to the elements, whichelements react with a noble metal to form a noble metal element alloythat is deleterious to the container, the free energy of the followingchemical reaction being always greater than zero and the partialpressure of oxygen being effective such that the oxide does not reduceaccording to the following reaction: MO, (in glass solution) M (in noblemetal solution, failed part) -1- x/2 (gas at P where M is an oxideforming element of a glass batch ingredient used to make high qualityglass articles, O is oxygen, X is the fraction or integer describing theoxide stoichiometry, and P is the partial pressure of oxygen, the changein free energy being expressed by the following equation:

where AF is the change in free energy for the reaction,

AF,(MO,) is the standard state Gibbs free energy of formation of theoxide, M0,, at a constant temperature; a is the thermodynamic activityof the oxide, M0, in glass solution; (1 is the thermodynamic activity ofthe element, M, in Pt solution at the point of Pt part failure; R is thegas constant equal to about 1.98717 cal/mole deg; and T is temperaturein degrees Kelvin. I

The present invention provides a method of melting glass lasercompositions with a minimum of platinum inclusions and a minimum ofdissolved platinum content in the resultant glass laser material, theprocess comprising melting the laser glass composition in a containerhaving at least 50% by weight of platinum in a reducing atmosphere witha buffered gas mixture, the atmosphere having a minimum level of thepartial pressure of oxygen that is equivalent to a range of about to 10'atmospheres for a temperature range of about 981F. to 3,321F. for asilicate glass; the partial pressure of oxygen of l0 atmospheres being aminimum at the low temperature of about 981F. and the partial pressureof 10'*' being a minimum at the high temperature of about 3,321F., theminimum level of the partial pressure of oxygen, expressed as log P atthe temperature used to melt the glass, being set forth below:

where the minimum levels of log P are determined according to thefollowing equation:

in which AF is the change in free energy; AF", is the standard stateGibbs free energy of formation of SiO which is 145,6OO cal/mole at2,600F.;

R is the universal gas constant equal to about 1.98717 cal/mole deg;

T is the temperature at which the. glass is melted in degrees Kelvinwhich is 1,700 at 2,600F.;

a is 0.15; a is 10 and P is the partial pressure of oxygen.

It is highly desirable to melt glass and form highquality glass articlesin a platinum container with no attack of the container. In accordancewith the present invention, this is accomplished by controlling thepartial pressure of oxygen above a minimum amount so that metal oxidesin the glass are not reduced to the metal which, in turn, would reactwith platinum to form a metal platinum alloy, the melting of the glassbeing accomplished in a reducing atmosphere in which the partialpressure of oxygen is no greater than about 10' with a buffered gasmixture such as CO /Nl-1 CO /H Hzo/Hg, Hzo/Nfls, and

It is preferred that the glasses (including laser glasses) of thepresent invention be lithium silicate glasses, although other silicateglasses are useful in making high-quality articles accordingto thepresent invention. Generally M is silicon in the preceding equationdescribing the change in free energy.

It is preferred that the glass composition be a lithium oxide-calciumoxide-aluminum oxide silicate composition containing at least about 8mole percent lithia. In this case, excellent high-quality articles,particularly glass laser articles, are made by melting the-compositionand controlling the partial pressure of oxygen so that it is no greaterthan about 10* atmospheres to thereby eliminate attack of the platinumcontainer, and the partial pressure of oxygen is also greater than about10' atmospheres at 2,600F-. to keep the platinum inclusions and thedissolved platinum content in the glass to a minimum. It is preferredthat the above glass (and particularly the laser glass) have thefollowing ingredients in approximate mole percent:

INGREDIENTS PERCENT SiO 45 to Li O 15 to 35 C210 k to 30 M1 0 1/10 to 2A1 0 0 to 8 TABLE 1 GLASS COMPOSITIONS Glass No. Component Mole Fraction1 510 0.785 BaO 0.014 Na O 0.080 K 0 0.1 12 Nd O 0.010 2 S10 0.785 A1 00.014 Sb O 0.002

TAB E v.1, 100.011.11 53 GLASS COMPOSITIONS Glass No. Mole FractionComponent 8210 0.014 ZnO 0.017 L1 0.023 Na O 0.068 K 0 0.1 13 Nap, 0.0103 2 0.791 T10, 0.004 Sb 0 0.005 BaO 0.018 PbO 0.006 Na,0 0.045 K 0 0.126Nd O 0.006 4 SiO, 0.480 B 0 0.174 A1 0 0.073 AS203 0.073 8210 0.255 Nd O0.013 5 510 0.593 A1 0; 0.025 K 0 2.12 X CeO 1.6 10 F11 0, 9.4 X 10- NaO 9.42 X 10- Li O 0.275 N0 0 0.005 C110 0100 0 810 0.762 A1 0, 0017 Sb O0.002 B110 0.023 ZnO 0.017 L1 0 0.023 Na O 0.077 K 0 0.080 Nd O 0010 7SiO, 0.792 A1 0 0.010 8110 0.022 ZnO 0.012 L1 0 0022 M1 0 0080 K 0 0.053Nd O; 0.007 8 S10, 0.773 smo, 0.002 BaO 0.1 13 Na O 0.100 K 0 0.013

Also suitable is the following glass composition given in approximateparts by weight:

510 67.17 C210 1080 Na,0 15.93 1420 .19 10,0 .75 $11.03 A510, .311 N0 04.78

The term noble metal failure or platinum failure" occurs in the case ofplatinum l ifa liquid phase were to be formed (2) if an intermetallicalloy compound is formed or (3) there is a large increase in theplatinum lattice parameter (d spacing) that occurs on the addition of acontaminating metal element. Hence, the point of failure of platinumparts in contact with the glass during the melting operation can becalculated using thermodynamic data such as free energy data, theactivities of oxides in the glass, and the activities of thecorresponding metals in platinum. The lowest oxygen partial pressure onecan use for controlling the melting of the lithium oxide-calciumoxide-aluminum oxide silicate glass composition is generally about 10 at2,600F. 1t is desired, of course, to reduce the partial pressure ofoxygen as low as possible in order to minimize the partial pressure ofany PtO (gas) above the melt and the activity of the correspondingplatinum oxide complex of the glass. Hence, in accordance with theequations previously set forth, platinum failure can be expected tooccur somewhere below 10' atmospheres at 2,600F., and usually not until10' atmospheres at 2,600F. (l,700K.). Generally, in the above-describedlithium oxide-calcium oxidealuminum oxide silicate glass, platinum metalfailure does not occur until a lower P is reached within the glass andhence the critical element is the Si0 reduction reaction to Si. Thefollowing table gives partial pressure of oxygen below which there willgenerally be platinum failure caused by the formation of theplatinum-metal element alloy.

It can be seen that in the case of the lithium oxidecalciumoxidealuminum oxide silicate, the silica reduction is controlling thelower limit of the P lf silica were not present, other oxideconstituents would control the lower limit of P in accordance with thepresent invention, the thermodynamic calculations for the partialpressure of oxygen provided for platinum metal failure in contact withan LiO-CaOSiO -Nd O glass can be written. It can be seen that platinumfailure can be expressed from a chemical point of view by reaction 1.Silicon is used as an example.

. Si (X in failed Pt) 0 (gas, P SiO (X in glass solution); T l,700K.Reaction R (1 The P required for AF 4 0 is desired. AF AF", (SiO l37,93lcal/mole at l,7()OK. 2,600F.).

For glass composition No. 5, X 0.593 and the a has been experimentallydetermined; because failure was not observed above P l0- atm., let a0.15 for the calculation. The mole fractions of the glass constituentsare not being changed in .empirical data, the a at failure has beenestimated to be 10 at l,533K. These data could be temperature correctedfrom 1,533K. to I,700K. by assuming that Tlog a is not a function oftemperature. This can be demonstrated by considering the reaction:

Me (pure) Me (in Pt. soln) For Rx (2), AF AH TAS AS O because the degreeof disorder is similar between solids and between solids and liquids;therefore, AF is not a strong F (T). Also note:

A P RT in 2 glass.

Therefore, T log a is not a strongf(t). Using this relation, the a at1,700K. can be calculated knowing a at 1,533K. Consider a as an example:

(l,533l(.) log [a l,533l(.] (1,700K.) log [a 1,700K.] log [a I,700K.](1533/1700) (10*) 0.9 X 10 10 l45,600 (1.987) glo Therefore, P s 10"atm. is order for failure to occur.

The same calculation is done for the other oxides and metals. The dataused is presented in Table 2.

The other method of estimating the a at failure assumes that -l at.% ofthe metal (Me) can be dissolved in the Pt before failure would beexpected. The a is related to the mole fraction (X by the relation a 'yX where 'y is the activity coefficient as empirically determined. Table3 lists the data used in the previous equation to calculate P requiredfor failure and the obtained P values.

TABLE 2 CALCULATION OF P REQUIRED FOR Pt FAILURE ASSUMINQ T 1.700K.2,600F.

Ele-

ment glass AF f(MeO P Required for TABLE 3 It can be seen thatoutstanding high-quality glass articles can be formed from glass meltsby melting the glass in a noble metal container, having at least about50 and preferably about by weight of platinum, and controlling theatmosphere with a buffered glass mixture that is a reducing mixture andcontrolling the partial pressure of oxygen therein so that theglass-forming oxides do not reduce and form the glass-forming elementwhich, in turn, can attack the platinum or other noble metal in thecontainer. The controlling of the partial pressure of oxygen is suchthat it is not greater than about 10 atmospheres and for alithia-calcia-alumino silicate glass is at a value equivalent to that ofabout 10 atmospheres at 2,600F. The following table provides the methodof selecting other partial pressures of oxygen at other meltingtemperatures other than 2,600F. This table sets forth the minimum log Pvalues for a silicate glass (preferably the Li O- CaOAl O SiO glasses)at various temperatures from about 261F. to 3,32lF. that can be usedwithout platinum or noble metal failure:

Hence, at about 2,600F., the partial pressure of oxygen should be keptabove about 10'- atmospheres. At

about 2,421F., the controlling of the partial pressure of oxygen shouldbe at a level above about 10 to prevent platinum failure. Hence, thecritical value of P of about 10* atmospheres at about 2,42 1 F. isequivalent to about 10' atmospheres at about 2,60I or 2,600F.

In the present invention, a reducing atmosphere is defined as anyatmosphere maintaining a P s 10' atm.

When a silicate glass is melted generally a range of P of about 10 to 10atm. is preferred to provide a practical melting process. For somecommercial applications, it is highly preferred that the P be controlledat about l0 to 10' atm., generally the closer to the minimum about 10'atm., the greater the advantage.

CALCULATION OF P REOUIRED FOR I! FAILURE ASSUMING V In the presentinvention that provides outstanding glass laser articles such as rodsand discs, the controlled reducing atmosphere generally should be onethat is equivalent to about to about 10 atmospheres at 2,600F., althoughit is preferred that the partial pressure of oxygen be about 10 to 10atmospheres. For best results in a practical process, the partialpressure is about 10 to 10 atmospheres at 2,600F. to greatly reduce theamount of platinum inclusions in the resultant glass laser rod and avoidattack on the platinum container.

In one outstanding embodiment, partial pressure of the oxygen is about10" atmospheres at 2,600F. when using a lithiacalcia-alumina silicateglass laser composition comprising the following ingredients inapproximate mole percentages:

INGREDIENTS PERCENT SiO 45 to 75 Li O 15 to 35 CaO V; to 30 Nap, 1/10 to2 A1 0;, 0 to 8 COMPONENT WEIGHT PERCENT SiO At least 45 up to 80 A1 0Greater than 4 up to 40 L1 0 Greater than 5.5 up to Nd O At least aboutI up to 8 Particularly outstanding glass laser rods have been made fromglass laser compositions comprising the following approximate weightpercent:

WEIGHT PERCENT COMPONENT sio 77.16 A1201, 4.68 L120 15.07 M1 0, 3.09

As indicated above, glass laser compositions of US. Pat. Nos. 3,457,182and 3,471,409 to Lee and Rapp provide outstanding laser rods, and thedisclosures of these patents are hereby incorporated by reference.

Excellent results, for instance, have been obtained using the followingneodymium oxide doped glass having the following approximatecomposition:

INGRI'IDII'IN'I'S MOLE PERCENT s10, (10 A 1 0 2.5 Li O 27.5 C210 10 NdO; 0.5 C80 0. 16

INGREDIENTS MOLE PERCENT $10 45 m L 15 m 35 R0 ,1. to 30 Nap, 1/10 to 2A1 0, 0 m 8 wherein R0 is selected from the alkaline earth oxide groupconsisting of MgO, BeO, SrO, 'BaO, mixtures thereof, and mixtures of oneor more of the foregoing with CaO, where the total amount of L1 0 and R0is not substantially higher than about 50 mole percent;

INGREDIENTS MOLE PERCENT SiO 45 to 75 LI O l8 to 35 R 0 0.5 to 10 Nd O;0.1 to 2 A1 0 0 to 10 wherein R 0 is a member selected from the groupconsisting of Na O; K 0, Rb O, Cs O and mixtures thereof.

By melting glass laser rods in a platinum container under reducingconditions, excellent glass laser rods have been obtained by controllingthe partial pressure of oxygen, such controlling of the partial pressureof oxygen being used successfully to reduce platinum inclusions, reducethe amount of dissolved platinum and also avoid attack of the platinumcontainer.

The present invention can be used in the apparatus and methods describedin Chapman and LeSueur copending United States patent application Ser.No. 877,076, filed 11/17/69. The buffered gas mixture is used to producethe atmosphere over the glass melt and is used as'the inert gas thatdoes not attack or oxidize the platinum-lined container such as thefurnace. This application is also incorporated by reference.

Laser rods can be produced by a double melting step in which the partialpressure of oxygen is controlled, say, between about 10 to 10atmospheres in the first step. The second step is conducted bycontrolling the P at about 10" or 10 atmospheres. Apparently the firststep is effective in hindering the dissolution of Pt into the glass, andthe second step is effective in dissolving any platinum inclusions toreproducibly obtain high-quality glass laser articles.

The reducing conditions preferably are provided by a buffered gasmixture that comprises a CO /CO mixture that contains generally about upto 6 to around 9 or percent of CO based on the volume of the mixture.Other suitable buffered gas mixtures are co /M1 CO /H H O/H H O/NH and HO/CO generally containing about 2 up to 5 to as high as 10% andpreferably about 6 to 8% of the reducing gas, such as CO, H O or NH;,.

Various modifications of the invention may be made under the patentstatutes without departing from the spirit of the invention.

We claim:

1. In a method of melting glass compositions in a container containingat least 50% by weight of platinum with a minimum of attack on thecontainer, the improvement comprising melting the glass in a reducingatmosphere in which the partial pressure of oxygen is no greater thanabout 10 atmospheres with a buffered gas mixture of the group consistingof CO /NH CO /H H O/l-I H O/NH H O/CO and CO /CO.

2. A method of melting glass to provide high-quality glass articles in aplatinum container without any attack on the container by the formationof a platinum alloy from the glass melt, the method comprising:

a. heating molten glass in the container in a reducing atmosphere inwhich the partial pressure of oxygen is no greater than about 10"atmospheres; and

b. controlling the reducing atmosphere with a buffcred gas mixture thatmaintains the partial pressure of oxygen above the amount at which theoxides of a glass-forming element in the glass are reduced to theelements, which elements react with a noble metal to form a noble metalelement alloy that is deleterious to the container.

3. A method as described in claim 2 in which the buffered gas mixture isCO /CO.

ered gas mixture is CO /NH 5. A method as defined in claim 2 in whichthe buffered gas mixture is CO /H 6. A method as defined in claim 2 inwhich the buffered gas mixture is H O/l-l 7. A method of melting glassto provide high-quality glass-articles with no platinum-alloy attack,the method comprising heating molten glass in a platinumcontainingcontainer in a reducing atmosphere with a buffered gas mixture of thegroup consisting of CO /CO, CO /NH CO /H H O/H H O/NH and H O/CO, thebuffered gas mixture controlling the partial pressure of oxygen to alevel not greater than about 10 atmospheres by adjusting and maintainingthe partial pressure of oxygen above the amount at which the metaloxides in the glass are reduced to the metal, which metal reacts withplatinum to form a metalplatinum alloy that is deleterious to thecontainer.

8. A method as defined in claim 7 in which the glass is a silicate glassand the buffered gas mixture is CO /CO.

9. A method as defined in claim 7 in which the glass is a lithiumsilicate glass composition containing at least about 8 mole percentlithia.

10. A method as defined in claim 7 in which the glass composition is alithium oxide-calcium oxidealuminum oxide silicate glass containing atleast about 8 mole percent lithia, and the partial pressure of oxygen iscontrolled so that it is not greater than 10 atmospheres at 2,600F.

11. A method as defined in claim 7 in which .the glass is a silicateglass and the reducing gas is CO, H O or Nl-l I 12. A method as definedin claim 7 in which the glass composition is a lithium oxide-calciumoxidealuminum oxide silicate glass containing at least about 8 molepercent lithia, and the buffered gas mixture is CO /CO containing about6 to 8 percent by volume CO.

13. A method as defined in claim 7 in which the buffered gas mixture isCO /NH

1. IN A METHOD OF MELTING GLASS COMPOSITION IN A CONTAINER CONTAINING ATLEAST 50% BY WEIGHT OF PLATINUM WITH A MINIMUM OF ATTACK ON THECONTAINER, THE IMPROVEMENT COMPRISING MELTING THE GLASS IN A REDUCINGATMOSPHERE IN WHICH THE PARTIAL PRESSURE OF OXYGEN IS NO GREATER THANABOUT 10**-3 ATMOSPHERES WITH ABUFFERED GAS MIXTURE OF THE GROUPCONSISTING OF CO2/NH3, CO2/H2, H2O/H2, H2O/NH3, H2O/CO AND CO2/CO
 2. Amethod of melting glass to provide high-quality glass articles in aplatinum container without any attack on the container by the formationof a platinum alloy from the glass melt, the method comprising: a.heating molten glass in the container in a reducing atmosphere in whichthe partial pressure of oxygen is no greater than about 10 3atmospheres; and b. controlling the reducing atmosphere with a bufferedgas mixture that maintains the partial pressure of oxygen above theamount at which the oxides of a glass-forming element in the glass arereduced to the elements, which elements react with a noble metal to forma noble metal element alloy that is deleterious to the container.
 3. Amethod as described in claim 2 in which the buffered gas mixture isCO2/CO.
 4. A method as defined in claim 2 in which the buffered gasmixture is CO2/NH3.
 5. A method as defined in claim 2 in which thebuffered gas mixture is CO2/H2.
 6. A method as defined in claim 2 inwhich the buffered gas mixture is H2O/H2.
 7. A method of melting glassto provide high-quality glass articles with no platinum-alloy attack,the method comprising heating molten glass in a platinum-containingcontainer in a reducing atmosphere with a buffered gas mixture of thegroup consisting of CO2/CO, CO2/NH3, CO2/H2, H2O/H2, H2O/NH3 and H2O/CO,the buffered gas mixture controlling the partial pressure of oxygen to alevel not greater than about 10 3 atmospheres by adjusting andmaintaining the partial pressure of oxygen above the amount at which themetal oxides in the glass are reduced to the metal, which metal reactswith platinum to form a metal-platinum alloy that is deleterious to thecontainer.
 8. A method as defined in claim 7 in which the glass is asilicate glass and the buffered gas mixture is CO2/CO.
 9. A method asdefined in claim 7 in which the glass is a lithium silicate glasscomposition containing at least about 8 mole percent lithia.
 10. Amethod as defined in claim 7 in which the glass composition is a lithiumoxide-calcium oxide-aluminum oxide silicate glass containing at leastabout 8 mole percent lithia, and the partial pressure of oxygen iscontrolled so that it is not greater than 10 3 atmospheres at 2,600*F.11. A method as defined in claim 7 in which the glass is a silicateglass and the reducing gas is CO, H2O or NH3.
 12. A method as defined inclaim 7 in which the glass composition is a lithium oxide-calciumoxide-aluminum oxide silicate glass containing at least about 8 molepercent lithia, and the buffered gas mixture is CO2/CO containing about6 to 8 percent by volume CO.
 13. A method as defined in claim 7 in whichthe buffered gas mixture is CO2/NH3.